Venezuelan President Hugo Chavez is heading back to Cuba for a third cancer surgery after naming his vice-president as his choice to lead the country if the illness cuts short his presidency.

Chavez's announcement on Saturday night unleashed new uncertainty about the country's future, and his supporters poured into city plazas across the nation to pray for his recovery from what appears to be an aggressive type of cancer.

Some wiped tears, while others held photos of him and chanted in unison: "Ooh-Ah! Chavez isn't going away!"

Chavez acknowledged the seriousness of his health situation in a televised address, saying for the first time that if he suffers complications Vice-President Nicolas Maduro should be elected as Venezuela's leader to continue his socialist movement.

Several outside medical experts said that based on Chavez's account of his condition and his treatment so far, they doubt the cancer can be cured.

Chavez said he hasn't given up.

"With the grace of God, we'll come out victorious," said Chavez, who held up a crucifix and kissed it during his Saturday night appearance.

Maduro to fill in temporarily

The 58-year-old president is still scheduled to be sworn in for a new six-year term Jan. 10. He has been in office for nearly 14 years, since 1999.

"There are risks. Who can deny it?" Chavez said, seated at the presidential palace beside Maduro and other aides. "In any circumstance, we should guarantee the advance of the Bolivarian Revolution."

Chavez, who won re-election on Oct. 7, said he would undergo surgery in Havana in the coming days. Lawmakers on Sunday voted unanimously to grant him permission to leave the country for the operation.

During the session at the National Assembly, opposition lawmakers agreed to Chavez's request and also said that Maduro should take on his duties during his temporary absence, as the constitution specifies.

'He's not asking for permission to leave his duties. The chief of this revolution is Hugo Chavez.'—National Assembly President Diosdado Cabello

Opposition lawmaker Julio Borges criticized the incomplete information that has been released about Chavez's cancer, saying: "Venezuela has a right to know the truth."

Throughout his treatment, Chavez has kept secret various details about his illness, including the precise location of the tumors and the type of cancer. He has said he travels to Cuba for treatment because his cancer was diagnosed by doctors there.

National Assembly President Diosdado Cabello said there are no plans at this time for Chavez to cede power, even temporarily, as president.

"He's not asking for permission to leave his duties," Cabello said. "The chief of this revolution is Hugo Chavez."

Cabello chided opposition politicians for questioning how forthcoming Chavez has been about his illness, likening them to "Komodo dragons."

Some of the pro-Chavez lawmakers cried and their voices cracked with emotion as they praised him and wished him a full recovery. They chanted, "Onward, commander!"

Under the Venezuelan constitution, vice-president Maduro would automatically fill in as president on a temporary basis should Chavez be unable to finish the current term concluding in early January.

But the constitution also says that if a president-elect dies before taking office, a new election should be held within 30 days. In the meantime, the president of the National Assembly is to be in charge of the government.

Chavez's supporters gather

More than 1,000 of Chavez's supporters gathered on Sunday in Plaza Bolivar in Caracas to show solidarity, many wearing his movement's red T-shirts while a marching band played.

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Supporters of Hugo Chavez gather Sunday at Simon Bolivar square in Caracas, Venezuela. Chavez is to return to Cuba for another surgery in his battle against cancer. (Fernando Llano/Associated Press)

State television showed Chavez's supporters congregating in city squares on Sunday and joining hands to pray for his health. In downtown Caracas, some expressed optimism that Chavez would pull through it. Others said they weren't sure.

"I love Chavez, and I'm worried," said Leonardo Chirinos, a construction worker. "We don't know what's going to happen, but I trust that the revolution is going to continue on, no matter what happens."

Chavez called his relapse a "new battle." It will be his third operation to remove cancerous tissue in about a year and a half.

The president underwent surgery for an unspecified type of pelvic cancer in Cuba in June 2011, after an earlier operation for a pelvic abscess. He had another cancer surgery last February after a tumour appeared in the same area. He has also undergone chemotherapy and radiation treatments.

Chavez said in July that tests showed he was cancer-free. But he had recently reduced his public appearances, and he made his most recent trip to Cuba on Nov. 27, saying he would receive hyperbaric oxygen treatment. Such treatment is regularly used to help heal tissues damaged by radiation treatment.

Chavez said that while in Cuba tests detected the recurrence of cancer.